LHRH neurons, which reside within the hypothalamus, are central to the regulation of the reproductive neuroendocrine axis. Activation of LHRH neurons is a key event in triggering the pituitary LH surge, which is responsible for causing ovulation, the hallmark event of the ovulatory cycle. Our focus is to examine the regulation of LHRH neuronal activation associated with the ovulatory LH surge. The expression of the immediate early gene product, cFos, is used as a marker of stimulated neuronal activity within LHRH neurons and their afferents. Brain tissue is collected from animals during the ovulatory LH surge and at other times of the rat estrous cycle. Immunocytochemistry and in situ hybridization are used to mark activated neurons, identify their phenotype and provide an indirect assessment of their level of activity. Tract-tracing techniques are used to determine the projections of activated populations of neurons. Populations of interest are those that project to the area of LHRH cell bodies or to areas known to be involved in the regulation of GnRH activity. Studies focus on the role of estrogens (i.e., their positive feedback effects) in altering neuronal function in preparation for LHRH neuronal activation associated with the ovulatory LH surge. Using neuroanatomical and pharmacological approaches, we are determining whether populations of neurons within the preoptic area, such as dopamine or neurotensin, play a role in transducing estrogens' effects on GnRH activity. Another goal of these studies is to examine whether immediate early gene expression in LHRH neurons is involved in the changes in gene expression that follow neuronal activation. This research project is generating new information about the reproductive neuroendocrine axis and, specifically, about the neuronal mechanisms involved in activation of LHRH neurons in association with this 640016ritical physiological event during the reproductive cycle. These studies provide the framework for using similar techniques in nonhuman primates to understand LHRH activation during the menstrual cycle.